Poly(hydroxyalkanoic acids), also known as polyhydroxyalkanoates (PHA), have gained great attention in the plastics industry because they can be produced from renewable monomers and are biodegradable. Because of these factors, these polymers have been proposed for use in a broad range of applications. Poly(hydroxyalkanoic acids) include polylactide resins (also known as polylactic acid or PLA), which are now available commercially. These resins can be produced from annually renewable resources such as corn, rice or other sugar- or starch-producing plants. In addition, PLA resins are compostable. For these reasons there is significant interest in substituting PLA into applications in which oil-based thermoplastic materials have conventionally been used.
However, physical limitations such as brittleness and slow crystallization may limit the applications of PHA resins. These limitations may prevent easy conversion of PHA into articles that have an acceptable degree of flexibility and toughness for many applications. Extruded amorphous sheeting may also be too brittle for handling in continuous moving equipment without breakage. Manufacturers and customers that use PHA to make a variety of articles are interested in improved processability and cycle times for articles made from this material.
Numerous impact modifiers have been developed in the past to improve the toughness of poly(hydroxyalkanoic acids). For example, Japanese Patent 9316310 discloses a poly(lactic acid) resin composition comprising a modified olefin compound as an impact modifier. US Patent Application Publication 2006/0173133 discloses a toughened poly(hydroxyalkanoic acid) composition wherein an ethylene ester copolymer (e.g., a terpolymer having copolymerized units of ethylene, butyl acrylate and glycidyl methacrylate (EBAGMA)) is used as an impact modifier.
Plasticizers are substances which, when added to another material, make that material softer and more flexible. Generally, this means that there is an increase in flexibility and workability, brought about by a decrease in the glass-transition temperature, Tg, of the composition.
Common plasticizers include phthalates such as diisobutyl phthalate, dibutyl phthalate, and benzylbutyl phthalate; adipates, including di-2-ethylhexyl adipate; trimellitates, including tris-2-ethylhexyl trimellitate; and phosphates, including tri-e-ethylhexyl phosphate. However, the use of some of these plasticizers has been curtailed due to potential toxicity issues. Polyester plasticizers have also been used, generally based on condensation products of propanediol or butanediol with adipic acid or phthalic anhydride, and therefore may exhibit very high viscosities which subsequently cause processing problems in blending with other polymers. Plasticizers are disclosed, for example, in D. F. Cadogan and C. J. Howick in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons, Inc., New York, Dec. 4, 2000, DOI: 10.1002/0471238961.1612011903010415.a01 and in the Handbook of Plasticizers; Edited by: Wypych, George; 2004 ChemTec Publishing; Chapter 11.
There is a desire to provide biodegradable plasticizers or plasticizers from renewable source, for natural polymers having improved or equivalent material properties to those provided by traditional, non-renewably sourced materials.
US Patent Application Publication 2009/0131566 discloses compositions comprising a PHA such as poly(lactic acid) modified with plasticizer and optionally a nucleator, an impact modifier, or both.
It is known in the art that each plasticizer has different behavior and the same plasticizer may behave differently with different polymers. Thus the type and concentration of plasticizer can be selected to improve the performance of a specific polymer. For example the behavior of polytrimethylene ether glycol when added to poly(lactic acid) polymer is different from the behavior of polyethylene glycol when added to PLA. Such differences are disclosed, for example, in Polymer Engineering and Science, 39, 1303, 1999.